A switching power converter transfers power from an input supply voltage source to one or more output voltage supply sources. In general, this is done by using the input supply to transfer energy to a storage element and then transferring the stored energy in a controlled manner to the output. In most commonly used switching power converters the energy is stored in some form of inductor; the exception being a class of converter known as a “charge pump” which stores energy in a capacitor. There are a variety of different switching converter topologies (circuits) such as buck, flyback, forward, sepic, half bridge, full bridge, and the like with the fundamental difference being how the inductive components are used to store the energy.
The flyback converter topology is commonly used in low- to mid-power switching converter designs because the low component count typically results in a small and inexpensive design. For example, these converters are commonly used in broadband access devices, such as digital subscriber loop (DSL), power over Ethernet (POE), cable modems, and the like. Advantageously, the flyback converter topology is well understood to be the simplest form of switching power converter design—requiring the minimum number of magnetic and rectifier components—that still provides input to output isolation. This simplicity and the associated low cost ensure that the flyback converter is likely the most commonly-used switching converter topology notwithstanding a few limitations.
Referring to FIG. 1, a single output flyback converter 10 with a conventional rectifier is illustrated. The flyback converter 10 includes a voltage source, Vin, 11, an output voltage, Vout, 12, a switch 13, a transformer 14, a diode 15, and a capacitor 16. When the switch 13 is on, the primary of the transformer 14 is directly connected to the input voltage source 11. This results in an increase of magnetic flux in the transformer 14. The voltage across the secondary winding is negative, so the diode 15 is reverse-biased (i.e., blocked). The output capacitor 16 supplies energy to the output load. When the switch 13 is off, the energy stored in the transformer 14 is transferred to the output of the converter 10.
Referring to FIGS. 2 and 3, it is also well known in the art that flyback converters 20,30 can be configured in a variety of ways to provide multiple output voltages. For example, FIG. 2 illustrates a dual output flyback converter 20 including a transformer 24 with parallel windings. Alternatively as illustrated in FIG. 3, one common approach is to utilize a flyback transformer 34 (or coupled inductor) with a “stacked” secondary winding 35,36. The stacked winding 35,36 arrangement results in improved cross regulation between the multiple output voltages due to the (partial) commonality in winding conduction losses and leakage inductance induced effects.
Referring to FIG. 4, as is generally true with switching converter designs, the efficiency of a flyback converter can often be improved by replacing the conventional output rectifiers with metal-oxide-semiconductor field-effect transistor (MOSFET) devices operated as “synchronous” rectifiers. FIG. 4 illustrates a single output flyback converter 40 with a synchronous rectifier 42.
Currently, there does not exist stacked output windings (e.g., as illustrated in FIG. 3) combined with synchronous rectification (e.g., as illustrated in FIG. 4) in the flyback converter topology. State of the art converter designs either generate multiple outputs with synchronous rectification or they generate multiple outputs with a stacked secondary winding configuration but without the use of synchronous rectifiers. There are some examples where a stacked output winding is used together with hybrid rectification where the highest power output uses a synchronous rectifier and the remaining outputs use conventional rectifiers. For example, this is illustrated in “Power Over Ethernet-Sync Flyback Reference Design”, available at focus.ti.com/docs/toolsw/folders/print/pmp411.html, PMP411, from Texas Instruments of Dallas, Tex. However, this does not utilize a stacked winding configuration for generating more than one voltage output while applying synchronous rectification to all of the outputs.
The use of hybrid rectification is appropriate when the additional outputs are required to deliver only small amounts of power compared to the output with the highest output power requirement. However, there are other applications when the efficiency and cross regulation improvements resulting from the use of synchronous rectification are desired on all outputs, such as a dual output converter where the power delivered from each output is in the range of perhaps 1:1 to 1:3.
Referring to FIG. 5, circuit diagram of a dual output flyback converter 50 with a synchronous rectifier is illustrated. When synchronous rectification is applied to the dual output converter 50 with parallel windings 58 all rectifier MOSFETs 52,54 are typically placed in the low side of the windings 58. This results in a common source voltage for the entire group of rectifier MOSFETs 52,54 which in turn allows a common gate signal 56 to be used to control all of the rectifier MOSFET gates 52,54.
Referring to FIG. 6, however, when synchronous rectification is applied to variants of the classical stacked winding topology, a flyback converter 60 has the resulting required rectifier 62,64 control signals referenced to different MOSFET source voltages, i.e. there is no common reference signal between the rectifiers 62,64. Thus the complexity of the rectifier control function is substantially increased due to the need for multiple MOSFET control voltages; at the very least these voltages must be derived through some form of level shifting and/or multiple control voltage windings.
Furthermore, a review of the literature shows that the exact operation of self-driven synchronous rectifier, forced continuous-current mode (CCM), flyback converters is not well understood. For example, Zhang et al. state in “Design Considerations and Performance Evaluations of Synchronous Rectification in Flyback Converters”, IEEE Transactions On Power Electronics, Vol. 13, No. 3, May 1998, that the self driven mode is not possible in a flyback converter. Other references, such as Kollman, “Achieving High-Efficiency with a Multi-Output CCM Flyback Supply Using Self-Driven Synchronous Rectifiers”, 2003, Texas Instruments, available at focus.ti.com/lit/ml/slup204/slup204.pdf, illustrate practical working examples of flyback converters with self driven synchronous rectification while neglecting to discuss the essential contribution of the transformer leakage inductance towards the desired operation of this topology. However, as pointed out in Xie et al., “Current-Driven Synchronous Rectification Technique For Flyback Topology”, IEEE 32nd Annual Power Electronics Specialists Conference, Volume 1, 2001, without sufficient leakage inductance in the power carrying windings of the transformer the MOSFETs in a self driven synchronously rectified converter will fail to commutate properly. The failure to commutate results in simultaneous current flow in both the primary and secondary windings of the flyback transformer which in turn typically results in a catastrophic failure of one or more of the MOSFET switches.